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The Question

(Submitted June 30, 1997)

If neutrinos are massless what else could make the galaxies spin like that? I am 18 years old and next year I be a student in physics at University level.

The Answer

The context of your question is lost, but I presume by 'spin like that' you are referring to me fact that the speed at which galaxies spin is too fast to be held together by the gravity of all the stars that we can see.

The galactic missing mass, which provides additional gravity, is probably produced by some sort of 'dark matter' (things we do not see). This can be in the form of Massive Compact Halo Objects (MACHOs) which may be stars too small to glow brightly (or other, more bizarre objects) or it can be heavy particles, or shadow matter, or primordial black holes, or any of a number of other things.

MACHOs have been detected through their gravitational effect on light, although there is no definitive knowledge of exactly what they are.

David Palmer
for Ask an Astrophysicist

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